Ceria electrolyte for low-temperature sintering and solid oxide fuel cell using the same

ABSTRACT

Disclosed is a ceria electrolyte for a solid oxide fuel cell, which is a ceria (CeO 2 ) electrolyte configured such that either gadolinium (Gd) or samarium (Sm) is co-doped with ytterbium (Yb) and bismuth (Bi), wherein Bi is doped in an amount of 0.5 to 5 mol %, thus exhibiting low-temperature sintering properties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ceria electrolyte for low-temperaturesintering of a solid oxide fuel cell (SOFC) and, more particularly, to aceria (CeO₂) electrolyte and a solid oxide fuel cell using the same, inwhich the ceria electrolyte is configured such that either gadolinium(Gd) or samarium (Sm) is co-doped with ytterbium (Yb) and bismuth (Bi),thus exhibiting low-temperature sintering properties.

2. Description of the Related Art

Recently, in order to reduce the operating temperature of a solid oxidefuel cell (SOFC) to ensure the high-temperature stability thereof and toprevent the power of the solid oxide fuel cell from decreasing due tothe reduction in the operating temperature, high-performance materialsare being actively applied.

Currently widely useful as an electrolyte material for a unit cell for asolid oxide fuel cell is a zirconia-based electrolyte such as 8YSZ (8mol % Y₂O₃ stabilized ZrO₂).

Initially, with regard to unit cells using a zirconia-based electrolytesuch as 8YSZ, a cathode material contains a composite comprising LSM(Sr-doped LaMnO₃), as a pure electron conductor, and an electrolytematerial. Recently, however, to increase the power density (W/cm²) ofthe unit cell, the use of a mixed ionic and electronic conductor (MIEC)such as LSCF (Sr- & Co-doped LaFeO₃), having superior oxygen ionizationcatalytic properties and high electrical conductivity even at lowtemperatures, is drastically increasing as the cathode material.

However, most MIEC cathode materials such as LSCF are problematicbecause they may react with the zirconia electrolyte at the interfacetherebetween in the thermal treatment temperature range of the cathode,undesirably forming non-conductive reaction products. Hence, in order toprevent the MIEC cathode such as LSCF and the zirconia electrolyte fromreacting at high temperatures, a buffer layer (BL) is additionallyprovided between the cathode and the electrolyte.

In particular, a ceria-based electrolyte (Gd- or Sm-doped CeO₂)comprising pure CeO₂ and 5 to 10 mol % of Gd₂O₃ or Sm₂O₃ has high oxygenionic conductivity and does not react with MIEC cathodes, and is therebywidely utilized as a material for a buffer layer, which is interposedbetween the zirconia (ZrO₂)-based electrolyte membrane of the solidoxide fuel cell and the MIEC cathode layer so as to prevent theproduction of a reaction product between the electrolyte and thecathode.

Useful as the buffer layer, a ceria-based electrolyte is characterizedby forming an isomorphous solid solution with a zirconia electrolyte ata high temperature of 1300° C. or higher, in which the solid solution,formed at a high temperature through mutual diffusion, has very lowionic conductivity, consequently deteriorating the power of the unitcell.

The fabrication of a unit cell using a ceria-based buffer layer includestwo types of methods, one method including forming an anode support, azirconia-based electrolyte and a ceria electrolyte into a laminatedmolded body that is then co-sintered, and the other method includingforming an anode support and a zirconia electrolyte into a laminatedmolded body that is then sintered, coating the surface of theelectrolyte membrane with a ceria buffer layer, and then thermallytreating it.

Thus, when the unit cell is manufactured by co-sintering the ceria-basedelectrolyte and the zirconia electrolyte, co-sintering at a temperatureof 1300° C. or lower is required, but the zirconia electrolyte has to besubjected to a sintering temperature of 1350° C. or higher in order toattain the dense microstructure of 95% or more typically required ofelectrolytes.

Also, when the ceria-based buffer layer is formed by preparing a denseelectrolyte through sintering at 1350° C. or higher and coating andthermally treating the surface of the electrolyte, the already-sinteredelectrolyte membrane does not shrink, undesirably making it difficult toform a dense ceria-based electrolyte and increasing the processing timeand cost due to the additional coating and thermal treatment.

Supposing that the thermal treatment temperature of the ceria-basedelectrolyte used as the buffer layer is lowered to the thermal treatmenttemperature of the cathode, the ceria-based electrolyte and the cathodemay be thermally treated simultaneously, thus reducing the processingtime and ensuring a dense microstructure even at a low temperature,thereby improving the power characteristics of the unit cell.

Therefore, techniques for decreasing the thermal treatment temperatureof the ceria-based electrolyte to 1150° C., corresponding to the thermaltreatment temperature of the cathode, are required.

CITATION LIST Patent Literature

-   Patent Document 1: Korean Patent Application Publication No.    10-2013-0040640-   Patent Document 2: Korean Patent Application Publication No.    10-2013-0099704

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems encountered in the related art, and an object of thepresent invention is to provide a ceria electrolyte, wherein aconventional Gd-doped CeO₂ (GDC) electrolyte or an Sm-doped CeO₂ (SDC)electrolyte is further doped with Bi to decrease the thermal treatmenttemperature thereof to 1150° C., which corresponds to the thermaltreatment temperature of a cathode, and is simultaneously doped with asmall amount of Yb to prevent the average cation radius from increasingdue to the additional doping with Bi and to prevent the ionicconductivity from decreasing, thereby lowering the sintering temperaturebelow that of typical ceria electrolytes.

In order to accomplish the above object, the present invention providesa ceria (CeO₂) electrolyte for low-temperature sintering, configuredsuch that either gadolinium (Gd) or samarium (Sm) is co-doped withytterbium (Yb) and bismuth (Bi) to exhibit low-temperature sinteringproperties.

The ceria (CeO₂) electrolyte may have an average cation radius of 0.98to 0.99 Å.

The ceria (CeO₂) electrolyte may be configured such that gadolinium(Gd), ytterbium (Yb) and bismuth (Bi) are co-doped to exhibitlow-temperature sintering properties, and has a composition of ChemicalFormula 1 below.Gd_(x)Yb_(y)Bi_(z)Ce_(1-x-y-z)O_(2−δ)  [Chemical Formula 1]

-   -   0.05≤X≤0.15, 0.005≤Y≤0.05, 0.005≤Z≤0.05, 0.06≤X+Y+Z≤0.25,        δ=(X+Y+Z)/2

The ceria (CeO₂) electrolyte, comprising Gd, Yb and Bi, which areco-doped, may have an average cation radius of 0.98 to 0.99 Å.

The ceria (CeO₂) electrolyte may be configured such that samarium (Sm),ytterbium (Yb) and bismuth (Bi) are co-doped to exhibit low-temperaturesintering properties, and has a composition of Chemical Formula 2 below.Sm_(x)Yb_(y)Bi_(z)Ce_(1-x-y-z)O_(2−δ)  [Chemical Formula 2]

-   -   0.1≤X≤0.17, 0.005≤Y≤0.05, 0.005≤Z≤0.05, 0.11≤X+Y+Z≤0.27,        δ=(X+Y+Z)/2

In addition, the present invention provides a solid oxide fuel cell,comprising: an anode; a zirconia electrolyte, formed throughco-sintering with the anode; a ceria buffer layer, formed by coating asurface of the zirconia electrolyte with the above ceria electrolyte andperforming thermal treatment at 1100 to 1200° C.; and a cathode, formedthrough coating and thermal treatment on a surface of the ceria bufferlayer.

The zirconia electrolyte may be a fluorite-type stabilizedzirconia-based electrolyte and may have a composition of ChemicalFormula 3 below.(Re₂O₃)_(x)(ZrO₂)_(1-x)  [Chemical Formula 3]

-   -   Re=at least one element selected from among Y, Sc, Yb, Ce, Gd,        and Sm, 0.04≤X≤0.11

The cathode may comprise a mixed conductor having a perovskite(ABO₃)-based crystal structure with both electronic and oxygen ionicconductivities, and may have a composition of Chemical Formula 4 below.ABO₃  [Chemical Formula 4]

-   -   A=at least one element selected from among La, Sm, Pr, Ba, Sr,        and Ca    -   B=at least one element selected from among Fe, Co, Ni, and Cu    -   A and B have a content ratio of 0.95≤A/B≤1

The cathode may comprise a mixed conductor having a double-layerperovskite (ABC₂O_(5+δ))-based crystal structure with both electronicand oxygen ionic conductivities, and may have a composition of ChemicalFormula 5 below.ABC₂O_(5+δ)  [Chemical Formula 5]

-   -   A=at least one element selected from among La, Sm, Nd, and Pr    -   B=at least one element selected from among Ba, Sr, and Ca    -   C=at least one element selected from among Co, Fe, Ni, Cu, and        Mn

The cathode may comprise 50 to 70 wt % of a mixed conductor having aperovskite (ABO₃)-based crystal structure or a mixed conductor having adouble-layer perovskite (ABC₂O_(5+δ))-based crystal structure and 30 to50 wt % of a ceria-based electrolyte, and the ceria-based electrolytemay have a composition of Chemical Formula 1 below.Gd_(x)Yb_(y)Bi_(z)Ce_(1-x-y-z)O_(2−δ)  [Chemical Formula 1]

-   -   0.05≤X≤0.15, 0.005≤Y≤0.05, 0.005≤Z≤0.05, 0.06≤X+Y+Z≤0.25,        δ=(X+Y+Z)/2

The cathode may comprise 50 to 70 wt % of a mixed conductor having aperovskite (ABO₃)-based crystal structure or a mixed conductor having adouble-layer perovskite (ABC₂O_(5+δ))-based crystal structure and 30 to50 wt % of a ceria-based electrolyte, and the ceria-based electrolytemay have a composition of Chemical Formula 2 below.Sm_(x)Yb_(y)Bi_(z)Ce_(1-x-y-z)O_(2−δ)  [Chemical Formula 2]

-   -   0.1≤X≤0.17, 0.005≤Y≤0.05, 0.005≤Z≤0.05, 0.11≤X+Y+Z≤0.27,        δ=(X+Y+Z)/2

The cathode may comprise 50 to 70 wt % of a mixed conductor having aperovskite (ABO₃)-based crystal structure or a mixed conductor having adouble-layer perovskite (ABC₂O_(5+δ))-based crystal structure and 30 to50 wt % of a ceria-based electrolyte, and the ceria-based electrolytemay have a composition of Chemical Formula 6 below.Re_(x)Ce_(1-x)O_(2−δ)  [Chemical Formula 6]

-   -   Re=at least one element selected from among Gd, Sm, Y, Nd, and        Pr, 0.05≤X≤0.2, δ=X/2

The anode may comprise 30 to 50 wt % of a fluorite-type stabilizedzirconia-based electrolyte and 50 to 70 wt % of NiO.

The solid oxide fuel cell may have a unit cell configuration of ananode-supported cell (ASC), an electrolyte-supported cell (ESC), ametal-supported cell (MSC), or a segmented-type cell.

The unit cell may be provided in a planar-type, tubular-type orflat-tube-type form.

According to the present invention, a ceria electrolyte forlow-temperature sintering is a ceria (CeO₂) electrolyte imparted withlow-temperature sintering properties by co-doping either Gd or Sm withYb and Bi, and can exhibit an average cation radius similar to that of aconventional GDC electrolyte or SDC electrolyte, thereby ensuring a highsintering density of 95% or more even at a sintering temperature of1150° C. while maintaining the high oxygen ionic conductivity of theconventional GDC electrolyte or SDC electrolyte.

According to the present invention, a solid oxide fuel cell using theceria electrolyte for low-temperature sintering is configured such thatthe ceria-based electrolyte can be firmly adhered to a dense electrolytemembrane, which is sintered at 1350° C. or higher by lowering thethermal treatment temperature of the ceria-based electrolyte to 1150° C.or less, corresponding to the thermal treatment temperature of acathode.

Furthermore, a ceria buffer layer and a MIEC cathode such as LSCF can besimultaneously thermally treated after being continuously applied, thusshortening the processing time and enabling the fabrication of a unitcell having high power density.

The aforementioned effects are set forth to illustrate, but are not tobe construed as limiting the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a graph illustrating the average cation radius of acommercially available GDC electrolyte;

FIG. 2 is a graph illustrating the average cation radius of a Yb-dopedGDC electrolyte;

FIG. 3 is a graph illustrating the average cation radius of a Bi-dopedGDC electrolyte;

FIG. 4 is a graph illustrating the average cation radius depending onthe Bi composition that is doped in the ceria electrolyte according toan embodiment of the present invention;

FIG. 5 is a graph illustrating the average cation radius of acommercially available SDC electrolyte;

FIG. 6 is a graph illustrating the average cation radius of a Bi-dopedSDC electrolyte;

FIG. 7 is a graph illustrating the average cation radius of a Yb-dopedSDC electrolyte;

FIG. 8 is a graph illustrating the sintering density depending on anincrease in the amount of doped Bi and an increase in the sinteringtemperature in the GDC electrolyte synthesized using a solid-statereaction process according to an embodiment of the present invention;

FIG. 9 is a graph illustrating the shrinkage rate depending on anincrease in the amount of doped Bi and an increase in the sinteringtemperature in the GDC electrolyte synthesized using a solid-statereaction process according to an embodiment of the present invention;

FIG. 10 is a graph illustrating the shrinkage rate in the Bi/Yb co-dopedceria electrolyte compositions sintered at 1200° C. for 5 hr aftersynthesis through a solid-state reaction process according to anembodiment of the present invention;

FIG. 11 is a graph illustrating the results of measurement of the ionicconductivity depending on the operating temperature using a DCfour-terminal process in the ceria electrolyte compositions sintered at1200° C. for 5 hr after synthesis through a solid-state reaction processaccording to an embodiment of the present invention;

FIG. 12 is a graph illustrating the sintering shrinkage rate dependingon the sintering temperature in the CE-3 electrolyte synthesized throughco-precipitation according to an embodiment of the present invention anda commercially available GDC electrolyte, which are molded in the samemanner;

FIG. 13 is a graph illustrating the sintering density depending on thesintering temperature in the CE-3 electrolyte synthesized throughco-precipitation according to an embodiment of the present invention andthe commercially available GDC electrolyte, which are molded in the samemanner;

FIG. 14 illustrates scanning electron microscope (SEM) images of themicrostructures depending on the sintering temperature in the CE-3electrolyte synthesized through co-precipitation according to anembodiment of the present invention and the commercially available GDCelectrolyte, which are molded in the same manner;

FIG. 15 is a graph illustrating the crystal structures, obtained throughX-ray diffractive analysis, of the CE-3 electrolyte synthesized throughco-precipitation according to an embodiment of the present invention andthe commercially available GDC electrolyte, which are molded in the samemanner and then sintered at 1200° C.;

FIG. 16 is a graph illustrating the results of measurement of the ionicconductivity depending on the operating temperature using a DCfour-terminal process in the CE-3 electrolyte, which is synthesizedthrough co-precipitation and then sintered at 1200° C., 1300° C. and1400° C. according to an embodiment of the present invention;

FIG. 17 is a graph illustrating the results of measurement of the ionicconductivity depending on the operating temperature using a DCfour-terminal process in the CE-3 electrolyte synthesized throughco-precipitation according to an embodiment of the present invention andthe commercially available GDC electrolyte, which are sintered at 1150°C. and 1300° C., respectively;

FIG. 18 illustrates SEM images of the microstructures of the fracturesurfaces of the ESC-1 and ESC-2 unit cells fabricated according to anembodiment of the present invention;

FIG. 19 is a graph illustrating the power density and voltage dependingon the current density when the ESC-1 and ESC-2 unit cells fabricatedaccording to an embodiment of the present invention are operated at 850°C.;

FIG. 20 is a graph illustrating the sintering density depending on anincrease in the amount of doped Bi and an increase in the sinteringtemperature in the SBC electrolyte synthesized using a solid-statereaction process according to an embodiment of the present invention;

FIG. 21 is a graph illustrating the shrinkage rate depending on anincrease in the amount of doped Bi and an increase in the sinteringtemperature in the SBC electrolyte synthesized using a solid-statereaction process according to an embodiment of the present invention;

FIG. 22 is a graph illustrating the sintering shrinkage rate dependingon the sintering temperature in the typical SDC electrolyte and the SYBCelectrolyte, which are synthesized using a citrate process and thenmolded in the same manner according to an embodiment of the presentinvention;

FIG. 23 is a graph illustrating the porosity depending on the sinteringtemperature in the typical SDC electrolyte and the SYBC electrolyte,which are synthesized using a citrate process and then molded in thesame manner according to an embodiment of the present invention;

FIG. 24 is a graph illustrating the sintering density depending on thesintering temperature in the typical SDC electrolyte and the SYBCelectrolyte, which are synthesized using a citrate process and thenmolded in the same manner according to an embodiment of the presentinvention;

FIG. 25 illustrates SEM images of the microstructures depending on thesintering temperature in the typical SDC electrolyte and the SYBCelectrolyte, which are synthesized using a citrate process and thenmolded in the same manner according to an embodiment of the presentinvention;

FIG. 26 is a graph illustrating the crystal structures, obtained throughX-ray diffractive analysis, of the typical SDC electrolyte and the SYBCelectrolyte, which are synthesized using a citrate process, molded inthe same manner and then sintered at 1100° C. according to an embodimentof the present invention;

FIG. 27 is a graph illustrating the results of measurement of the ionicconductivity depending on the operating temperature using a DCfour-terminal process in the typical SDC electrolyte, which issynthesized using a citrate process and then sintered at 1100 to 1400°C. according to an embodiment of the present invention;

FIG. 28 is a graph illustrating the results of measurement of the ionicconductivity depending on the operating temperature using a DCfour-terminal process in the SYBC electrolyte, which is synthesizedusing a citrate process and then sintered at 1100 to 1400° C. accordingto an embodiment of the present invention;

FIG. 29 is a graph illustrating the results of measurement of the ionicconductivity at 750° C. in the SDC and SYBC electrolytes, which aresynthesized using a citrate process and then sintered at 1100 to 1400°C. according to an embodiment of the present invention;

FIG. 30 is a graph illustrating the ionic conductivity of the SDC andSYBC electrolytes, which are synthesized using a citrate process andsintered at 1100 and 1200° C., which are considered low sinteringtemperatures, according to an embodiment of the present invention;

FIG. 31 illustrates images of the microstructures of ESC including theLSM cathode and ESC including the LSCF cathode and the SYBC electrolyteas the buffer layer, according to the present invention;

FIG. 32 is a graph illustrating the power density of ESC including theLSM cathode, operated at 750° C., 800° C. and 850° C., according to thepresent invention; and

FIG. 33 is a graph illustrating the power density of ESC including theLSCF cathode and the SYBC electrolyte as the buffer layer, operated at750° C., 800° C. and 850° C., according to the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Hereinafter, a detailed description will be given of preferredembodiments of the present invention with reference to the appendeddrawings. The embodiments of the present invention are provided to morefully describe the technical spirit of the present invention to thoseskilled in the art, and may be modified in various ways and are notconstrued as limiting the present invention. Rather, these embodimentsare provided to complete the present disclosure and to fully deliver thetechnical spirit of the present invention to those skilled in the art.As used herein, the term “and/or” may include any one of the listeditems and any combination of one or more thereof. Throughout thedrawings, the same reference numerals refer to the same or like parts.Furthermore, various parts and areas in the drawings are schematicallydepicted. Hence, the technical spirit of the present invention is notlimited by the relative sizes or intervals shown in the drawings.

According to the present invention, a ceria electrolyte forlow-temperature sintering is configured such that either gadolinium (Gd)or samarium (Sm) is co-doped with ytterbium (Yb) and bismuth (Bi), andmay thus ensure low-temperature sintering properties.

The average cation radius of the ceria (CeO₂) electrolyte, configuredsuch that either Gd or Sm is co-doped with Yb and Bi, falls in the rangeof 0.98 to 0.99 Å.

In an embodiment of the present invention, the ceria electrolyte forlow-temperature sintering is a ceria (CeO₂) electrolyte configured suchthat Gd, Yb and Bi are co-doped to exhibit low-temperature sinteringproperties, and may have the composition of Chemical Formula 1 below.Gd_(x)Yb_(y)Bi_(z)Ce_(1-x-y-z)O_(2−δ)  [Chemical Formula 1]

-   -   0.05≤X≤0.15, 0.005≤Y≤0.05, 0.005≤Z≤0.05, 0.06≤X+Y+Z≤0.25,        δ=(X+Y+Z)/2

The currently commercially available ceria-based electrolyte isrepresented by Gd_(x)Ce_(1-x)O_(2−δ). When some Ce⁴⁺ main lattice ionsare doped with Gd³⁺, oxygen vacancies are formed in the lattice and areused as the diffusion path of oxygen ions. The Gd-doped ceriaelectrolyte is referred to as a GDC electrolyte.

As the amount of doped Gd in the GDC electrolyte is increased, ionicconductivity rises. In the case where Gd is doped in an excessiveamount, oxygen vacancies are combined, undesirably deteriorating ionicconductivity. Typically, the amount of doped Gd in the ceria (CeO₂)electrolyte falls in the range of 10 to 20 mol %. Given the above amountof doped Gd, the highest ionic conductivity may result. Individualresearch groups report different compositions that manifest the highestionic conductivity.

The ceria electrolyte for low-temperature sintering according to thepresent invention includes Bi as an additional doping element forensuring the low-temperature sintering properties of the GDC electrolyteand maintaining the high ionic conductivity thereof, and Yb as anadditional doping element for preventing the average cation radius fromincreasing due to the introduction of Bi. The cation radii of Gd³⁺,Bi³⁺, Ce⁴⁺ and Yb³⁺ are 1.053 Å, 1.17 Å, 0.97 Å, and 0.985 Å,respectively.

The composition of the GDC electrolyte known to have the highest ionicconductivity is represented by Gd_(x)Ce_(1-x)O_(2-(x/2)), where X is 0.1to 0.2, and the average radius of all the cations in the abovecomposition may be shown in FIG. 1. Based on the calculated results, thecation radii of the commercially available GDC electrolytes may fall inthe range of about 0.978 to 0.987 (Å).

Changes in the cation radius of the Yb-doped GDC electrolyte(Gd_(2-x)Yb_(x)Ce_(0.8)O_(1.9)) are shown in FIG. 2. Some Gd ions aredoped with Yb having a relatively small ionic radius, and the averagecation radius is decreased with an increase in the doped amount thereof,which can be confirmed through calculation.

On the other hand, changes in the cation radius of the Bi-doped GDCelectrolyte (Gd_(0.15)Bi_(x)Ce_(0.85-x)O_(2-(0.15+x)/2)) are shown inFIG. 3, in which the average cation radius may become large with anincrease in the amount of doped Bi, which has a relatively large ionicradius compared to Gd and Ce, which can be confirmed throughcalculation.

Specifically, when Bi is additionally doped to ensure thelow-temperature sintering properties of a conventional GDC electrolyte,the average cation radius is increased. This increase in the cationradius may narrow the diffusion path (oxygen vacancies) of oxygen ions,undesirably impeding the efficient conduction of oxygen ions.

In the present invention, hence, Yb, which has an ionic radius smallerthan that of Gd, may be co-doped with the goal of suppressing anincrease in the average cation radius due to doping with Bi, which hasan ionic radius greater than Gd.

FIG. 4 is a graph illustrating the average cation radius depending onthe Bi composition that is doped in the ceria electrolyte according toan embodiment of the present invention. The average cation radius of theceria (CeO₂) electrolyte, comprising Gd, Yb and Bi, which are co-doped,may fall in the range of 0.98 to 0.99 Å.

In an embodiment of the present invention, the ceria electrolyte forlow-temperature sintering is a ceria (CeO₂) electrolyte configured suchthat Sm, Yb and Bi are co-doped to thus ensure low-temperature sinteringproperties, and may have the composition of Chemical Formula 2 below.Sm_(x)Yb_(y)Bi_(z)Ce_(1-x-y-z)O_(2−δ)  [Chemical Formula 2]

-   -   0.1≤X≤0.17, 0.005≤Y≤0.05, 0.005≤Z≤0.05, 0.11≤X+Y+Z≤0.27,        δ=(X+Y+Z)/2

The currently commercially available ceria-based electrolyte isrepresented by Sm_(x)Ce_(1-x)O_(2−δ). When some Ce⁴⁺ main lattice ionsare doped with Sm³⁺, oxygen vacancies are formed in the lattice and areused as the diffusion path of oxygen ions. The Sm-doped ceriaelectrolyte is referred to as an SDC electrolyte.

The ceria electrolyte for low-temperature sintering according to thepresent invention includes Bi as an additional doping element forensuring the low-temperature sintering properties of the SDC electrolyteand maintaining high ionic conductivity, and Yb as an additional dopingelement for suppressing an increase in the average cation radius due tothe introduction of Bi. The cation radii of Sm³⁺, Bi³⁺, Ce⁴⁺ and Yb³⁺are 1.079 Å, 1.17 Å, 0.97 Å, and 0.985 Å, respectively.

The composition of the SDC electrolyte known to have the highest ionicconductivity is represented by Sm_(x)Ce_(1-x)O_(2-(x/2)), where X is 0.1to 0.2, and the average radius of all the cations in the abovecomposition may be shown in FIG. 5. Based on the calculated results, thecation radii of the commercially available SDC electrolytes may fall inthe range of about 0.98 to 0.99 (Å).

In the case where some Sm ions are substituted with Bi to ensure thelow-temperature sintering properties of the SDC electrolyte, that is, inthe case of the Bi-doped SDC electrolyte(Sm_(0.15)Bi_(x)Ce_(0.85-x)O_(2-(0.15+x)/2)), the changes in the cationradius are shown in FIG. 6. As Bi, which has a relatively large ionicradius compared to Sm and Ce, is doped in a larger amount, the averagecation radius is increased, which can be confirmed through calculation.When X, which indicates the amount of Bi, is 0.02 or more, the averagecation radius is increased to 0.99 (Å) or more.

When Bi is additionally doped to ensure the low-temperature sinteringproperties of a conventional SDC electrolyte, the average cation radiusis increased. This increase in the cationic radius may narrow thediffusion path (oxygen vacancies) of oxygen ions, thus impeding theconduction of oxygen ions.

On the other hand, changes in the cation radius of the Yb-doped SDCelectrolyte (Sm_(2-x)Yb_(x)Ce_(0.8)O_(1.9)) are shown in FIG. 7, inwhich some Sm ions are doped with Yb, which has a relatively small ionicradius, and the average cation radius is decreased with an increase inthe doped amount thereof, which can be confirmed through calculation.

Hence, Yb, which has an ionic radius smaller than Sm, may be co-doped tosuppress an increase in the average cation radius due to the doping withBi, which has an ionic radius greater than Sm.

In addition, the present invention addresses a solid oxide fuel cellusing the ceria electrolyte for low-temperature sintering as above,comprising an anode, a zirconia electrolyte, a ceria buffer layer, and acathode.

The anode may comprise 30 to 50 wt % of a fluorite-type stabilizedzirconia-based electrolyte and 50 to 70 wt % of NiO.

The zirconia electrolyte, which may be formed through co-sintering withthe anode, may be a fluorite-type stabilized zirconia-based electrolyte,and may have the composition of Chemical Formula 3 below.(Re₂O₃)_(x)(ZrO₂)_(1-x)  [Chemical Formula 3]

-   -   Re=at least one element selected from among Y, Sc, Yb, Ce, Gd,        and Sm, 0.04≤X≤0.11

The ceria buffer layer may be formed by coating the surface of thezirconia electrolyte with the ceria electrolyte for low-temperaturesintering and then thermally treating it at 1100 to 1200° C.

The cathode may be formed through coating and thermal treatment on thesurface of the ceria buffer layer, and may be composed of a mixedconductor having a perovskite (ABO₃)-based crystal structure with bothelectronic and oxygen ionic conductivities, and may have the compositionof Chemical Formula 4 below.ABO₃  [Chemical Formula 4]

-   -   A=at least one element selected from among La, Sm, Pr, Ba, Sr,        and Ca    -   B=at least one element selected from among Fe, Co, Ni, and Cu    -   The content ratio of A and B is 0.95≤A/B≤1

Also, the cathode may comprise a mixed conductor having a double-layerperovskite (ABC₂O_(5+δ))-based crystal structure with both electronicand oxygen ionic conductivities, and may have the composition ofChemical Formula 5 below.ABC₂O_(5+δ)  [Chemical Formula 5]

-   -   A=at least one element selected from among La, Sm, Nd, and Pr    -   B=at least one element selected from among Ba, Sr, and Ca    -   C=at least one element selected from among Co, Fe, Ni, Cu, and        Mn

Also, the cathode may comprise 50 to 70 wt % of the mixed conductorhaving a perovskite (ABO₃)-based crystal structure or the mixedconductor having a double-layer perovskite (ABC₂O_(5+δ))-based crystalstructure, and 30 to 50 wt % of a ceria-based electrolyte, in which theceria-based electrolyte may have the composition of Chemical Formula 1below.Gd_(x)Yb_(y)Bi_(z)Ce_(1-x-y-z)O_(2−δ)  [Chemical Formula 1]

-   -   0.05≤X≤0.15, 0.005≤Y≤0.05, 0.005≤Z≤0.05, 0.06≤X+Y+Z≤0.25,        =(X+Y+Z)/2

Also, the cathode may comprise 50 to 70 wt % of the mixed conductorhaving a perovskite (ABO₃)-based crystal structure or the mixedconductor having a double-layer perovskite (ABC₂O_(5+δ))-based crystalstructure, and 30 to 50 wt % of a ceria-based electrolyte, in which theceria-based electrolyte may have the composition of Chemical Formula 2below.Sm_(x)Yb_(y)Bi_(z)Ce_(1-x-y-z)O_(2−δ)  [Chemical Formula 2]

-   -   0.1≤X≤0.17, 0.005≤Y≤0.05, 0.005≤Z≤0.05, 0.11≤X+Y+Z≤0.27,        δ=(X+Y+Z)/2

In this way, when the ceria-based electrolyte contained in the cathodeof the solid oxide fuel cell has the same composition as the ceria-basedelectrolyte for the buffer layer, the thermal treatment time may beshortened, and a dense microstructure may be ensured even at lowtemperatures, thus improving the power characteristics of the unit cell.

Also, the cathode may comprise 50 to 70 wt % of the mixed conductorhaving a perovskite (ABO₃)-based crystal structure or the mixedconductor having a double-layer perovskite (ABC₂O_(5+δ))-based crystalstructure, and 30 to 50 wt % of a ceria-based electrolyte, in which theceria-based electrolyte may have the composition of Chemical Formula 6below.Re_(x)Ce_(1-x)O_(2−δ)  [Chemical Formula 6]

-   -   Re=at least one element selected from among Gd, Sm, Y, Nd, and        Pr, 0.05≤X≤0.2, δ=X/2

The solid oxide fuel cell may have a unit cell configuration of, forexample, an anode-supported cell (ASC), an electrolyte-supported cell(ESC), a metal-supported cell (MSC), or a segmented-type cell, and sucha unit cell may be provided in a planar-type, tubular-type orflat-tube-type form.

The tubular-type single cell facilitates gas sealing and exhibitssuperior mechanical properties in terms of the structure thereof, andmay thus be used to manufacture a large stack having power outputranging from tens of to hundreds of kW, but suffers from low powerdensity per unit volume. The planar-type single cell has high powerdensity per unit volume but is disadvantageous in terms of stacksealing, high reactivity between individual parts, and poor mechanicalstability of the cell. This type is favorably employed in an auxiliarypower unit (APU) having power output of ones of kW for transport andbuildings of 10 kW or more, as well as home appliances of 1 kW or lessrequiring high power density. The tubular-type single cell is suitablefor use in an anode-supported cell, a cathode-supported cell, or asegment-type cell, and the planner-type cell may be fabricated in theform of an anode-supported cell, an electrolyte-supported cell or ametal-supported cell. The flat-tube-type single cell may be currentlyutilized in home appliances based on an anode-supported cell or in powergeneration based on a segmented-type cell.

Below is a description of embodiments of the present invention, madethrough examples and test examples.

1. Evaluation of Bi Doping Effect in GDC Electrolyte 1-1. CompositionDesign

In order to evaluate the Bi doping effect, the compositions of pure GDCelectrolytes (Gd_(x)Ce_(1-x)O_(2-x/2), X=0.1, 0.2) and Gd/Bi co-dopedelectrolytes (Gd_(0.1)Bi_(x)Ce_(0.9-x)O_(2−δ), X=0.025, 0.05, 0.075,0.1, δ=(0.1+X)/2) were designed as shown in Table 1 below.

TABLE 1 Sample Composition 1: GDC-19 Gd_(0.1)Ce_(0.9)O_(1.95) 2: GDC-28Gd_(0.2)Ce_(0.8)O_(1.9) 3: X = 0.025Gd_(0.1)Bi_(0.025)Ce_(0.875)O_(1.9375) 4: X = 0.05Gd_(0.1)Bi_(0.05)Ce_(0.85)O_(1.925) 5: X = 0.075Gd_(0.1)Bi_(0.075)Ce_(0.825)O_(1.9125) 6: X = 0.1Gd_(0.1)Bi_(0.1)Ce_(0.8)O_(1.9)

1-2. Preparation of Electrolyte Sample

In order to synthesize the electrolytes having the compositions of Table1, Gd₂O₃, Bi₂O₃ and CeO₂ were used as raw materials, weighed so as to beadapted for the corresponding compositions, and then subjected to wetball mill mixing using an ethanol solvent and zirconia balls having adiameter of 5 mm as milling media.

The wet-mixed electrolyte slurries were dried at 75° C., and then moldedinto planar- and disk-type powder compacts through uniaxial pressing.

-   -   Planar-type molded body: width 40 mm, length 40 mm, thickness 4        mm    -   Disk-type molded body: diameter 27 mm, thickness 3 mm

The molded electrolyte samples were sintered at 1100 to 1400° C. for 5hr in air.

1-3. Evaluation of Density and Shrinkage Rate of Sintered Body

FIG. 8 illustrates the results of comparison of the sintering density ofthe GDC (Gd_(x)Ce_(1-x)O_(2-x/2), X=0.1, X=2), GDC-19 (X=0.1), andGDC-28 (X=0.2) electrolytes and the Gd/Bi co-doped electrolytes(Gd_(0.1)Bi_(x)Ce_(0.9-x)O_(2−δ), X=0.025, 0.05, 0.075, 0.1,δ=(0.1+X)/2), sintered at 1100° C., 1200° C., 1300° C. and 1400° C. for5 hr after synthesis through a solid-state reaction process using thecompositions of Table 1.

As seen in FIG. 8, when sintering was performed at 1100° C. and 1200° C.which are the low sintering temperatures of interest, the sinteringdensity was increased with an increase in the amount of Bi. Also, whenBi was used in an excessive amount (X=0.075, 0.1) under the condition ofa high sintering temperature of 1300° C., the sintering density waslower than at 1200° C.

FIG. 9 illustrates the results of calculation of the shrinkage rate ofthe electrolyte samples sintered at 1200° C. and 1300° C. correspondingto the results of FIG. 8, based on the following equation.

${{Shrinkage}\mspace{14mu}{rate}} = {\frac{\left( {{{diameter}\mspace{14mu}{of}\mspace{14mu}{molded}\mspace{14mu}{body}} - {{diameter}\mspace{14mu}{of}\mspace{14mu}{sintered}\mspace{14mu}{body}}} \right)}{{diameter}\mspace{14mu}{of}\mspace{14mu}{molded}\mspace{14mu}{body}} \times 100\mspace{11mu}(\%)}$

As shown in FIG. 9, the electrolyte samples sintered at 1200° C.exhibited a shrinkage rate that increased with an increase in the amountof Bi. When the volume of the electrolyte was decreased in this way, thesintering density thereof was increased. On the other hand, theshrinkage rate of the electrolyte samples sintered at 1300° C. wasincreased with an increase in the amount of Bi and was maximized in thecomposition where X is 0.05. Then, as the amount of Bi was increasedfurther, the shrinkage rate was somewhat decreased. This increase in theshrinkage rate was considered to be due to volume expansion caused byover-sintering, and the volume expansion resulted in lowered sinteringdensity, as shown in FIG. 8.

Thus, the Bi doping effect significantly contributes to thelow-temperature sintering of the GDC electrolyte, but the oppositeeffect may be shown when used in excess. Hence, the amount of Bipreferably falls in the range that does not exceed 5 mol % (X=0.05).

2. Evaluation of Bi/Yb Co-Doping Effect in GDC Electrolyte 2-1. Designof Composition of Ceria Electrolyte

In order to evaluate the Bi doping effect in the electrolyte in whichGd-doped ceria (GDC) is co-doped with Bi and Yb, the compositions of thechemical formulas shown in Table 2 below were designed.

TABLE 2 Sample Composition CE-1(Gd_(0.9)Yb_(0.1))_(0.15)Ce_(0.85)O_(1.925)Gd_(0.135)Yb_(0.015)Ce_(0.85)O_(1.925) CE-2(Gd_(0.9)Yb_(0.1))_(0.15)Bi_(0.01)Ce_(0.84)O_(1.92)Gd_(0.135)Yb_(0.015)Bi_(0.01)Ce_(0.84)O_(1.92) CE-3(Gd_(0.9)Yb_(0.1))_(0.15)Bi_(0.02)Ce_(0.83)O_(1.915)Gd_(0.135)Yb_(0.015)Bi_(0.02)Ce_(0.83)O_(1.915) CE-4(Gd_(0.9)Yb_(0.1))_(0.15)Bi_(0.03)Ce_(0.82)O_(1.91)Gd_(0.135)Yb_(0.015)Bi_(0.03)Ce_(0.82)O_(1.91) CE-5(Gd_(0.9)Yb_(0.1))_(0.15)Bi_(0.04)Ce_(0.81)O_(1.905)Gd_(0.135)Yb_(0.015)Bi_(0.04)Ce_(0.81)O_(1.905) CE-6(Gd_(0.9)Yb_(0.1))_(0.15)Bi_(0.05)Ce_(0.8)O_(1.9)Gd_(0.135)Yb_(0.015)Bi_(0.05)Ce_(0.8)O_(1.9)

FIG. 4 illustrates the results of calculation of the radii of all thecations of the electrolytes having the above compositions, which rangefrom 0.98 to 0.99 (Å) and are controlled within the average cation sizerange of conventional commercially available GDC electrolytes.

2-2. Preparation of Electrolyte Sample

In order to synthesize the ceria-based electrolytes having thecompositions of Table 2, Gd₂O₃, Yb₂O₃, Bi₂O₃ and CeO₂ were used as rawmaterials, weighed so as to be adapted for the correspondingcompositions, and then subjected to wet ball mill mixing using anethanol solvent and zirconia balls having a diameter of 5 mm as millingmedia.

The wet-mixed electrolyte slurries were dried at 75° C., and then moldedinto planar-type and disk-type powder compacts through uniaxialpressing.

-   -   Planar-type molded body: width 40 mm, length 40 mm, thickness 4        mm    -   Disk-type molded body: diameter 27 mm, thickness 3 mm

The molded electrolyte samples were sintered at 1200° C. for 5 hr inair. For comparative evaluation, an electrolyte sample was manufacturedusing a commercially available GDC (Gd_(0.1)Ce_(0.9)O_(1.95), LSA grade,made by Japan Anan Kasei) powder.

2-3. Evaluation of Density and Shrinkage Rate of Sintered Body

FIG. 10 illustrates the results of measurement of the shrinkage rate ofthe ceria electrolytes, which are co-doped with Yb and Bi shown in Table2 and sintered at 1200° C. for 5 hr. These electrolytes exhibited thesame shrinkage rate behavior as the electrolytes sintered at 1200° C. ofFIG. 9. When Bi was used in an amount of 5 mol % or less,low-temperature sintering properties of interest could be ensured.

Particularly as shown in FIG. 10, in the CE-3 to CE-6 electrolytesamples, synthesized using a solid-state reaction process and sinteredat 1200° C. for 5 hr and having a shrinkage rate of 10% or more, the useof Bi in an amount of 2 mol % or more is more preferable in terms ofassuring low-temperature sintering properties.

2-4. Evaluation of Ionic Conductivity

FIG. 11 illustrates changes in ionic conductivity depending ontemperature using a DC four-terminal process, in the CE-1 to CE-6planar-type electrolyte samples, sintered at 1200° C. for 5 hr andprocessed in the shape of a bar having a width of 2 mm, a length of 20mm and a height of 2 mm.

The ionic conductivity of the fluorite-type ceria- and zirconia-basedelectrolytes is typically known to increase as the concentration ofoxygen vacancies and the sintering density are increased and as theionic radius of the doped cation for forming oxygen vacancies becomessimilar to the radius of the main lattice ion (Ce⁴⁺ or Zr⁴⁺).

As shown in FIG. 11, as the amount of doped Bi was increased from CE-1to CE-4, the ionic conductivity was increased to approximate the ionicconductivity of the commercially available GDC electrolyte, which wassintered at 1400° C. This is considered to be because the sinteringdensity and the oxygen vacancies increased with an increase in theamount of Bi. Bi is present in the form of Bi³⁺ in the electrolytelattice, as in Gd³⁺ and Yb³⁺, thus forming oxygen vacancies.

In particular, as shown in FIG. 11, the use of Bi in an amount of 2 to 4mol % is more preferable, as in the CE-3 to CE-5 electrolyte sampleshaving an ionic conductivity of 0.02 S/cm or more at 700 to 800° C.,corresponding to the actual SOFC operating temperature.

2-5. Comparison with Commercially Available GDC Electrolyte

The CE-3 ceria electrolyte was synthesized using a wet co-precipitationprocess as a commercially available synthesis process, and water-solubleGd nitrate (Gd(NO₃)₃.6H₂O), Yb nitrate (Yb(NO₃)₃.6H₂O), Bi nitrate(Bi(NO₃)₃.5H₂O) and Ce nitrate (Ce(NO₃)₃.6H₂O) were dissolved in purewater, thus preparing a 0.25 M mixed nitrate aqueous solution.

The mixed nitrate was continuously stirred, and aqueous ammonia (NH₄OH),as a precipitating agent, was added dropwise at a rate of 100 cc/min tothe mixed nitrate using a metering pump to obtain a pH of 10, and theprecipitated amorphous metal hydride was washed five times with water,and the electrolyte powder was calcined at 970° C. for 3 hr to grow andcrystallize it, and then subjected to wet ball milling and drying, thussynthesizing a final CE-3 electrolyte powder.

The CE-3 powder synthesized through co-precipitation and the comparativeLSA grade powder, made by Anan Kasei, were molded into planar-type anddisk-type powder compacts through uniaxial pressing.

-   -   Planar-type molded body: width 40 mm, length 40 mm, thickness 4        mm    -   Disk-type molded body: diameter 27 mm, thickness 3 mm

The molded electrolyte samples were sintered at 1100 to 1400° C. for 5hr in air.

FIG. 12 illustrates the shrinkage rate depending on the sinteringtemperature in the CE-3 electrolyte, synthesized throughco-precipitation, and the comparative commercially available GDCelectrolyte made by Anan Kasei, which are sintered at the sametemperature. The CE-3 electrolyte exhibited the maximum shrinkage rateat 1200° C. but the shrinkage rate of the commercially available GDCincreased continuously with an increase in the sintering temperature.

FIG. 13 illustrates the sintering density depending on the sinteringtemperature in the CE-3 electrolyte synthesized through co-precipitationand the comparative commercially available GDC electrolyte made by AnanKasei, which were sintered at the same temperature. These results showthe same pattern as the results of evaluation of the shrinkage rate.Specifically, the shrinking and sintering of the CE-3 electrolyte werecompleted at 1200° C., and the growth of crystal grains andover-sintering thereof progressed with an increase in the sinteringtemperature, and the commercially available GDC was continuously shrunkand sintered up to 1400° C.

These results can be confirmed through the analysis of microstructuresbased on the SEM images of FIG. 14. The commercially available GDCsimultaneously underwent densification and growth of crystal grains withan increase in the sintering temperature, whereas the densification ofthe CE-3 electrolyte was almost completed at 1100° C., and thus only thegrowth of crystal grains progressed with an increase in the sinteringtemperature.

FIG. 15 illustrates the results of analysis of crystal structures,obtained using an X-ray diffractive analyzer, of the CE-3 electrolyteand the commercially available GDC electrolyte, which were sintered at1200° C. These two samples exhibited the same pattern, but the CE-3electrolyte showed high crystallinity in the relatively low sinteringtemperature range.

FIG. 16 illustrates the results of measurement of the ionic conductivityat 700 to 800° C., corresponding to the actual SOFC operatingtemperature, in the CE-3 electrolyte samples synthesized throughco-precipitation and sintered at 1200° C., 1300° C. and 1400° C. TheCE-3 electrolyte sample, sintered at 1200° C., at which the sinteringdensity and the shrinkage rate were the highest, exhibited the maximumionic conductivity at the same temperature. The ionic conductivity wasdecreased with an increase in the sintering temperature.

This is deemed to be because the sintering density was decreased due toover-sintering with an increase in the sintering temperature, and alsobecause oxygen vacancies were reduced and pores were formed due to theBi element, which was volatile at high temperature.

FIG. 17 illustrates the ionic conductivity depending on the operatingtemperature in the CE-3 electrolyte, synthesized throughco-precipitation and sintered at 1150° C., and the commerciallyavailable GDC electrolyte, sintered at 1300° C. As the operatingtemperature was increased above 700° C., the ionic conductivity of theCE-3 electrolyte was further increased.

2-6. Fabrication of Unit Cell

The power characteristics of a unit cell, manufactured by introducingthe CE-3 electrolyte powder synthesized through co-precipitation as thebuffer layer of the unit cell, were measured.

The unit cell was a type of electrolyte-supported cell (ESC), and wasmanufactured through a series of procedures including: molding ofelectrolyte support→sintering of electrolyte support→coating and thermaltreatment of anode→coating and thermal treatment of cathode.

A unit cell (ESC-1) including an LSM (Sr-doped LaMnO₃) cathode wasmanufactured through the above procedures, and a unit cell (ESC-2)including an LSCF (Sr- & Co-doped LaFeO₃) cathode was manufactured byadditionally performing coating and thermal treatment of the CE-3electrolyte, before the formation of the cathode.

Molding of Electrolyte Support:

A 10Sc1CeSZ (10 mol % Sc₂O₃+1 mol % CeO₂+89 mol % ZrO₂) electrolytepowder was subjected to uniaxial pressing, thus producing a disk-typemolded body having a diameter of 25 mm and a thickness of 2 mm.

Sintering of Electrolyte Support:

The disk-type electrolyte molded body was sintered at 1450° C. for 5 hrin air, and processed into an electrolyte support having a diameter of20 mm and a thickness of 300 μm.

Coating and Thermal Treatment of Anode:

The anode composite powder (NiO:10Sc1CeSZ=57:43 wt %) was prepared intoa printing paste, which was then applied on an anode having a diameterof 10 mm, and then thermally treated at 1250° C. for 3 hr in air.

Coating and Thermal Treatment of Buffer Layer:

As for the unit cell (ESC-2) including an LSCF cathode, the CE-3electrolyte powder synthesized through co-precipitation was applied inthe same manner as in the anode, and then thermally treated at 1150° C.for 3 hr in air.

Coating and Thermal Treatment of Cathode:

As the cathode materials for ESC-1 and ESC-2, the LSM(La_(0.7)Sr_(0.3)MnO₃) cathode and the LSCF(La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃) cathode were used. For ESC-1, thecathode composite (LSM:10Sc1CeSZ=60:40 wt %) and the LSM cathode werecontinuously coated and thermally treated at 1050° C. for 3 hr.

For ESC-2, the cathode composite (LSCF:CE-3=60:40 wt %) and the LSCFcathode were continuously coated and thermally treated at 1050° C. for 3hr.

In the present example, the thermal treatment temperature of the cathodewas set to 1050° C., but could vary in the range of 1000 to 1200° C.through control of the particle size of the cathode powder.

2-7. Evaluation of Power Characteristics of Unit Cell

FIG. 18 illustrates the results of analysis of microstructures of thefracture surfaces of ESC-1 and ESC-2, which show the microstructures ofthe dense electrolyte and the porous cathode. In particular, the CE-3buffer layer for ESC-2 manifested superior interfacial adhesion to theelectrolyte as well as high density, achieved through low-temperaturethermal treatment at 1150° C.

The power characteristics of ESC-1 and ESC-2 were evaluated at 850° C.,which is a typical ESC operating temperature, and air and hydrogen weresupplied at 150 cc/min and 50 cc/min to the cathode and the anode,respectively. FIG. 19 illustrates the power characteristics of ESC-1 andESC-2. The maximum power densities of ESC-1 and ESC-2 were respectively0.61 W/cm² and 0.81 W/cm², and respectively 0.58 W/cm² and 0.68 W/cm²under the current density condition of 1 A/cm².

Consequently, ESC-2 including the LSCF MIEC cathode and the CE-3 bufferlayer obtained through low-temperature thermal treatment exhibited themaximum power density that was increased by at least 30% at the sametemperature, compared to ESC-1 including the LSM cathode.

3. Evaluation of Bi Doping Effect in SDC Electrolyte 3-1. CompositionDesign

In order to evaluate the Bi doping effect, the compositions of pure SDCelectrolytes (Sm_(x)Ce_(1-x)O_(2-x/2), X=0.1, 0.2) and Sm/Bi co-dopedelectrolytes (Sm_(0.1)Bi_(x)Ce_(0.9-x)O_(2−δ), X=0.025, 0.05, 0.075,0.1, δ=(0.1+X)/2, SBC) were designed as shown in Table 3 below.

TABLE 3 Sample Composition 1: SDC-19 Sm_(0.1)Ce_(0.9)O_(1.95) 2: SDC-28Sm_(0.2)Ce_(0.8)O_(1.9) 3: X = 0.025Sm_(0.1)Bi_(0.025)Ce_(0.875)O_(1.9375) 4: X = 0.05Sm_(0.1)Bi_(0.05)Ce_(0.85)O_(1.925) 5: X = 0.075Sm_(0.1)Bi_(0.075)Ce_(0.825)O_(1.9125) 6: X = 0.1Sm_(0.1)Bi_(0.1)Ce_(0.8)O_(1.9)

3-2. Preparation of Electrolyte Sample

In order to synthesize the electrolytes having the compositions of Table3 through a solid-state reaction process, Sm₂O₃, Bi₂O₃ and CeO₂ wereused as raw materials, weighed so as to be adapted for the correspondingcompositions, and then subjected to wet ball mill mixing using anethanol solvent and zirconia balls having a diameter of 5 mm as millingmedia.

The wet-mixed electrolyte slurries were dried and then molded intoplanar-type and disk-type powder compacts through uniaxial pressing.

-   -   Planar-type molded body: width 40 mm, length 40 mm, thickness 4        mm    -   Disk-type molded body: diameter 27 mm, thickness 3 mm The molded        electrolyte samples were sintered at 1100 to 1400° C. for 5 hr        in air.

3-3. Evaluation of Shrinkage Rate, Porosity and Sintering Density ofSintered Body

FIG. 20 illustrates the results of measurement of the sintering densityin the SDC (Sm_(x)Ce_(1-x)O_(2-x/2), X=0.1, X=2), SDC-19 (X=0.1) andSDC-28 (X=0.2) electrolytes and the Sm/Bi co-doped electrolytes (SBC:Sm_(0.1)Bi_(x)Ce_(0.9-x)O_(2−δ), X=0.025, 0.05, 0.075, 0.1,δ=(0.1+X)/2), sintered at 1100° C., 1200° C., 1300° C. and 1400° C. for5 hr after synthesis through a solid-state reaction process using thecompositions of Table 3.

As shown in FIG. 20, when sintering was performed at 1100° C. and 1200°C., which are the low sintering temperatures of interest, the sinteringdensity was increased with an increase in the amount of Bi. In the casewhere Bi was used in an excessive amount (X=0.075, 0.1) at a highsintering temperature of 1300° C., the sintering density was decreasedcompared to when the sintering temperature was 1200° C., or the effectthereof was not further improved.

FIG. 21 illustrates the results of calculation of the shrinkage rate inthe electrolyte samples sintered at 1200° C. and 1300° C. correspondingto the results of FIG. 20, using the following equation.

${{Shrinkage}\mspace{14mu}{rate}} = {\frac{\left( {{{diameter}\mspace{14mu}{of}\mspace{14mu}{molded}\mspace{14mu}{body}} - {{diameter}\mspace{14mu}{of}\mspace{14mu}{sintered}\mspace{14mu}{body}}} \right)}{{diameter}\mspace{14mu}{of}\mspace{14mu}{molded}\mspace{14mu}{body}} \times 100\mspace{11mu}(\%)}$

As illustrated in FIG. 21, the electrolyte samples sintered at 1200° C.were increased in shrinkage rate with an increase in the amount of Bi.As the volume of the electrolyte was reduced, the sintering density ofthe electrolyte was increased. On the other hand, the shrinkage rate ofthe electrolyte samples sintered at 1300° C. was increased with anincrease in the amount of Bi and was maximized in the compositions whereX is 0.05 and 0.075. Also, as the amount of Bi was increased further,the shrinkage rate was somewhat decreased. It is considered that thisincrease in the shrinkage rate resulted from volume expansion due toover-sintering, and that the sintering density was decreased owing tothe volume expansion, as shown in FIG. 20.

Therefore, the Bi doping effect significantly contributes to thelow-temperature sintering of the SDC electrolyte. However, when Bi isused in an excessive amount, the opposite effect is shown. Hence, theamount of Bi preferably falls in the range that does not exceed 5 mol %(X=0.05).

4. Evaluation of Bi/Yb Co-Doping Effect in SDC Electrolyte 4-1. Designof Composition of Ceria Electrolyte

In order to evaluate the Bi doping effect in a typical SDC electrolyteand a Bi/Yb co-doped electrolyte (SYBC), the compositions represented bychemical formulas of Table 4 below were designed.

TABLE 4 Sample Composition SDC Sm_(0.2)Ce_(0.8)O_(1.9) SYBCSm_(0.16)Yb_(0.02)Bi_(0.02)Ce_(0.8)O_(1.9)

4-2. Preparation of Electrolyte Sample

The SDC and SYBC having the compositions of Table 4 were manufactured bydissolving a metal nitrate material at the corresponding compositionratio in pure water and then dissolving citric acid therein to give amixed aqueous solution, which was then stirred and heated at 250° C.,thus obtaining a concentrated solution.

The concentrated solution was burned at 500° C., and the burned SDC andSYBC powders were subjected to wet ball milling, dispersed, dried,thermally treated at 1000° C., and then again subjected to wet ballmilling.

The finally synthesized SDC and SYBC powders were molded intoplanar-type and disk-type powder compacts through uniaxial pressing.

-   -   Planar-type molded body: width 40 mm, length 40 mm, and        thickness 4 mm    -   Disk-type molded body: diameter 27 mm, thickness 3 mm

The molded electrolyte samples were sintered at 1100 to 1400° C. for 3hr in air.

4-3. Evaluation of Shrinkage Rate, Porosity and Sintering Density ofSintered Body

FIG. 22 illustrates the results of measurement of the shrinkage rate inthe SDC and SYBC sintered bodies, synthesized through a citrate processand sintered at 1100 to 1400° C. for 3 hr in air.

Based on the results of measurement of shrinkage rate, the SDCelectrolyte exhibited typical ceramic sintering behavior, in which theshrinkage rate is gradually decreased with an increase in the sinteringtemperature, whereas the SYBC electrolyte manifested the maximumshrinkage rate at a sintering temperature of 1200° C., and was reducedin the shrinkage rate due to over-sintering at 1300° C. or higher.

FIGS. 23 and 24 illustrate the results of evaluation of the porosity andsintering density using the Archimedes principle in the SDC and SYBCsintered bodies, prepared through a citrate process and sintered at 1100to 1400° C. for 3 hr in air.

In the evaluation of porosity, the porosity of the SDC electrolyte wasdrastically decreased due to the densification through sintering in thetemperature range of 1100 to 1300° C., and was slightly decreased in thehigh-temperature range of 1300 to 1400° C. The porosity of the SYBCelectrolyte was considerably reduced due to the densification throughsintering in the low-temperature range of 1100 to 1200° C., was slightlydecreased in the middle-temperature range of 1200 to 1300° C., and wasincreased due to over-sintering in the high-temperature range of 1300 to1400° C. Particularly the SYBC electrolyte had a porosity of 10% or lessafter sintering at 1200° C. for 3 hr, and thus could sufficiently ensurethe low-temperature sintering properties of interest.

The sintering density of SDC and SYBC exhibited results similar to theshrinkage rate behavior thereof and opposite to the porosity behaviorthereof. Consequently, the sintering density of the SYBC electrolytesintered at 1200° C. was equivalent to that of the SDC electrolytesintered at 1400° C., thus showing excellent low-temperature sinteringproperties of SYBC.

4-4. Evaluation of Microstructure of Sintered Body

FIG. 25 illustrates the results of SEM analysis of microstructures ofthe SDC and SYBC sintered bodies, sintered at 1100° C. and 1300° C. TheSDC sintered at 1100° C. was not densified, but the SYBC electrolytesintered at the same temperature was considerably densified. Thedensification and the growth of crystal grains in the SDC sintered at1300° C. progressed, and most crystal grains had a small size of 0.5 μmor less, whereas the growth of crystal grains sufficiently progressed inthe SYBC electrolyte sintered at the same temperature, thus formingcoarse crystal grains having a size of about 1 μm.

FIG. 26 is a graph illustrating the crystal structures, obtained throughX-ray diffractive analysis, of the typical SDC electrolyte and the SYBCelectrolyte, which were molded in the same manner and sintered at 1100°C., according to an embodiment of the present invention. The crystalstructures of SDC and SYBC sintered bodies sintered at 1100° C. wereanalyzed, and as shown in FIG. 26, both electrolytes manifested thefluorite-type crystal structure without any impurities or secondaryphase, and the crystallinity of the densified SYBC was relatively high.

4-5. Evaluation of Ionic Conductivity

In order to evaluate the ionic conductivity of the SDC and SYBCelectrolytes prepared through a citrate process and sintered in thecorresponding temperature range, the planar-type electrolyte sampleswere processed in the form of a bar shape having a width of 2 mm, alength of 20 mm and a height of 2 mm, and then changes in ionicconductivity thereof were measured at different temperatures using a DCfour-terminal process.

The ionic conductivity of the fluorite-type ceria- and zirconia-basedelectrolytes is generally known to increase as the concentration ofoxygen vacancies and the sintering density are increased and as theradius of the doped cation for forming oxygen vacancies becomes similarto the radius of the main lattice ion (Ce⁴⁺ or Zr⁴⁺).

FIG. 27 is a graph illustrating the results of measurement of the ionicconductivity at different operating temperatures using a DCfour-terminal process in the typical SDC electrolyte sintered at 1100 to1400° C. according to the embodiment of the invention. The ionicconductivity of the SDC electrolyte is directly correlated with thesintering density. As the sintering temperature increases, the increasein sintering density is almost the same as the increase in ionicconductivity.

FIG. 28 is a graph illustrating the results of measurement of the ionicconductivity at different operating temperatures using a DCfour-terminal process in the SYBC electrolyte sintered at 1100 to 1400°C. according to the embodiment of the invention. As for the SYBCelectrolyte having low-temperature sintering properties, all of theelectrolytes, other than the electrolyte sintered at 1100° C., exhibitedequivalent ionic conductivity results, whereby the ionic conductivityunder low-temperature sintering conditions was equivalent to that of theSDC sintered at high temperatures.

FIG. 29 is a graph illustrating the results of measurement of the ionicconductivity at 750° C. in the SDC and SYBC electrolytes sintered at1100 to 1400° C. according to the embodiment of the invention, and theseresults show the same pattern as the shrinkage rate results of FIG. 22.

FIG. 30 is a graph illustrating the ionic conductivity of the SDC andSYBC electrolytes sintered at 1100° C. and 1200° C., which are regardedas low sintering temperatures according to the embodiment of theinvention. The ionic conductivity of SYBC is remarkably high thanks tothe low-temperature sintering properties thereof, and thus SYBC isusable as a buffer layer through low-temperature sintering.

Consequently, the oxygen vacancies (O_(2−δ), δ=0.1) of SDC and SYBC arethe same as each other, and the high ionic conductivity of SYBC throughlow-temperature sintering was deemed to result from doping with Bi andYb, which show the same pattern as the results of shrinkage rate of FIG.22.

In particular, the SYBC electrolyte sintered at 1100° C. and 1200° C.,which are considered low sintering temperatures, as shown in FIG. 30,exhibited an ionic conductivity of 0.02 S/cm or higher at 750° C.,corresponding to the actual SOFC operating temperature, thussufficiently ensuring the low-temperature sintering properties ofinterest.

4-6. Fabrication of Unit Cell

The power characteristics of a unit cell including the SYBC electrolyteas the buffer layer of the unit cell were analyzed.

The single cell was fabricated through a series of procedures,including: uniaxial pressing of electrolyte support→sintering ofelectrolyte support (1450° C., 5 hr)→coating and thermal treatment ofanode (1300° C., 3 hr)→coating and thermal treatment of SYBC bufferlayer (1200° C., 3 hr)→coating and thermal treatment of LSCF cathode(1050° C., 3 hr).

Molding of electrolyte support: A 10Sc1CeSZ (10 mol % Sc₂O₃ & 1 mol %CeO₂ stabilized ZrO₂) electrolyte powder was subjected to uniaxialpressing using a mold having a diameter of 27 mm.

Sintering of electrolyte support: The 27 mm disk-type electrolyte moldedbody, obtained by uniaxial pressing, was sintered at 1450° C. for 5 hrin air and then mechanically processed, thus producing an electrolytesupport having a diameter of 20 mm and a thickness of 300 μm.

Coating and thermal treatment of anode: A composite powder(NiO:10Sc1CeSZ=58:42 wt %) having a specific surface area (BET) of 8 to10 m²/g was prepared into a paste, which was then applied on an anodehaving a diameter of 8 mm through screen printing, and then thermallytreated at 1300° C. for 3 hr in air.

Coating and thermal treatment of SYBC buffer layer: A SYBC electrolytepowder was prepared into a paste, which was then applied on SYBC havinga diameter of 8 mm, and then thermally treated at 1200° C. for 3 hr inair.

Coating and thermal treatment of cathode: Each of a fine compositepowder (LSCF:SYBC=60:40 wt %) having a specific surface area (BET) of 8to 10 m²/g and a coarse LSCF powder having a specific surface area (BET)of 4 to 6 m²/g was prepared into a paste, which was then applied on theCFL (cathode functional layer) and the CCC (cathode current collector)cathode having a diameter of 8 mm through continuous screen printing,and then thermally treated at 1050° C. for 3 hr in air.

For comparative evaluation, a single cell comprising LSM cathode(CFL=LSM (60):10Sc1CeSZ (40), CCC=LSM), without the SYBC buffer layer,was manufactured in the same manner as above.

4-7. Evaluation of Power Characteristics of Unit Cell

FIG. 31 illustrates the microstructures of the ESC including the LSMcathode and the ESC including the LSCF cathode and the SYBC electrolyteas the buffer layer, according to the present invention. Both kinds ofsingle cells had an electrolyte thickness of 300 μm.

FIG. 32 is a graph illustrating the power density when the ESC includingthe LSM cathode according to the present invention is operated at 750°C., 800° C. and 850° C., and FIG. 33 is a graph illustrating the powerdensity when the ESC including the LSCF cathode and the SYBC electrolyteas the buffer layer according to the present invention is operated at750° C., 800° C. and 850° C.

The single cell including the LSM cathode exhibited a maximum powerdensity of 0.65 W/cm² under the condition of a current density of about1.6 Å/cm² at 850° C. and a power density of about 0.55 W/cm² under thecondition of a rating voltage of 0.7 V.

Meanwhile, the single cell including the LSCF cathode and the SYBCbuffer layer exhibited a maximum power density of 0.95 W/cm² under thecondition of a current density of about 1.8 A/cm² at 850° C. and a powerdensity of about 0.85 W/cm² under the condition of a rating voltage of0.7 V. Accordingly, the power density was increased about 45%.

According to the present invention, the ceria-based electrolyte isuseful as a ceria-based buffer layer for preventing the interfacialreaction between the MIEC cathode and the zirconia-based electrolyte ina high-power solid oxide fuel cell. Compared to conventional ceria-basedbuffer layer materials including Sm-doped CeO₂ (SDC) or Gd-doped CeO₂(GDC), the ceria-based electrolyte of the invention can undergolow-temperature sintering, and has a controlled average cation radius,thus ensuring ionic conductivity equivalent to that of conventional SDCor GDC electrolyte material.

According to the present invention, the ceria electrolyte is configuredsuch that either Gd or Sm is co-doped with Bi and Yb, thus assuringlow-temperature sintering properties due to the introduction of Bi andcontrolling the average cation radius due to the introduction of Yb toassure oxygen ionic conductivity.

Moreover, the diffusion of the zirconia-based electrolyte and theceria-based electrolyte owing to high-temperature sintering can beprevented, and the buffer layer can be stably formed throughlow-temperature sintering. As the novel ceria-based buffer layer isintroduced, the power of the solid oxide fuel cell can be increased 30%or more.

As mentioned hereinbefore, although the preferred embodiments of thepresent invention have been disclosed herein and in the drawings, thoseskilled in the art will appreciate that various modifications, additionsand substitutions are possible, without departing from the scope andspirit of the invention as disclosed in the accompanying claims.

What is claimed is:
 1. A ceria (CeO₂) electrolyte for low-temperature sintering, suitable for use in a solid oxide fuel cell, comprising: gadolinium (Gd), ytterbium (Yb), and bismuth (Bi), which are simultaneously co-doped to exhibit a sintering density of 95% or more at a sintering temperature of 1200° C., wherein the ceria (CeO₂) electrolyte being simultaneously co-doped with Gd, the Yb, the Bi has an average cation radius of 0.98 to 0.99 Å and a composition of Chemical Formula 1 below: Gd_(x)Yb_(y)Bi_(z)Ce_(1-x-y-z)O_(2−δ)  [Chemical Formula 1]  0.05≤X≤0.15, 0.005≤Y≤0.05, 0.005≤Z≤0.05, 0.06≤X+Y+≤0.25, δ=(X+Y+Z)/2.
 2. A solid oxide fuel cell, comprising: an anode; a zirconia electrolyte, formed through co-sintering with the anode; a ceria buffer layer, formed by coating a surface of the zirconia electrolyte with the ceria electrolyte of claim 1 and performing thermal treatment at 1100 to 1200° C.; and a cathode, formed through coating and thermal treatment on a surface of the ceria buffer layer.
 3. The solid oxide fuel cell of claim 2, wherein the zirconia electrolyte is a fluorite-type stabilized zirconia-based electrolyte and has a composition of Chemical Formula 3 below: (Re₂O₃)_(x)(ZrO₂)_(1-x)  [Chemical Formula 3] Re=at least one element selected from among Y, Sc, Yb, Ce, Gd, and Sm, 0.04≤X≤0.11.
 4. The solid oxide fuel cell of claim 2, wherein the cathode comprises a mixed conductor having a perovskite (ABO₃)-based crystal structure with both electronic and oxygen ionic conductivities, and has a composition of Chemical Formula 4 below: ABO₃  [Chemical Formula 4] A=at least one element selected from among La, Sm, Pr, Ba, Sr, and Ca B=at least one element selected from among Fe, Co, Ni, and Cu A and B have a content ratio of 0.95≤A/B≤1.
 5. The solid oxide fuel cell of claim 2, wherein the cathode comprises a mixed conductor having a double-layer perovskite (ABC₂O_(5+δ))-based crystal structure with both electronic and oxygen ionic conductivities, and has a composition of Chemical Formula 5 below: ABC₂O_(5+δ)  [Chemical Formula 5] A=at least one element selected from among La, Sm, Nd, and Pr B=at least one element selected from among Ba, Sr, and Ca C=at least one element selected from among Co, Fe, Ni, Cu, and Mn.
 6. The solid oxide fuel cell of claim 2, wherein the cathode comprises 50 to 70 wt % of a mixed conductor having a perovskite (ABO₃)-based crystal structure or a mixed conductor having a double-layer perovskite (ABC₂O_(5+δ))-based crystal structure and 30 to 50 wt % of a ceria-based electrolyte.
 7. The solid oxide fuel cell of claim 2, wherein the anode comprises 30 to 50 wt % of a fluorite-type stabilized zirconia-based electrolyte and 50 to 70 wt % of NiO.
 8. The solid oxide fuel cell of claim 2, wherein the solid oxide fuel cell has a unit cell configuration of an anode-supported cell (ASC), an electrolyte-supported cell (ESC), a metal-supported cell (MSC), or a segmented-type cell.
 9. The solid oxide fuel cell of claim 8, wherein a unit cell is provided in a planar-type, tubular-type or flat-tube-type form. 